1. Purpose and Requirements of the Invention
A circuit was needed to measure conductance, with an output voltage proportional to conductance, over a five decade range, such as from 0.01 microSiemens (uS) to 1000 microSiemens (uS) or 0.1 microSiemens (uS) to 10,000 microSiemens (uS).
(Definitions of resistivity, conductance, and conductivity are given in Appendix II.)
The primary use of the circuit would be to measure the conductance of solutions, such as in those solutions containing biological growth media in a bacteriological sample, where an increase in the conductance of the solution indicates growth, or multiplication, of the bacteriological colonies. FIG. 1 illustrates schematically a typical chamber 10 containing a test solution with electrodes 1 and 2 within the solution. The electrodes are used to apply current to the test solution. More information about one type of such chambers and electrodes is available in the U.S. Pat. No. 4,072,578 to Cady et al.
1. One requirement of this type of measurement circuit is that the current used to measure the conductance of the solution must alternate in polarity and contain an insignificant component of net direct current in order to minimize polarization of, or electrically induced changes to, the measuring electrodes in the solution.
2. Another requirement is that the measurement of solution conductance must ignore the effects of electrode capacitance of the solution conductance (generally in solutions the inductive component is negligible) and only measure the dissipative component of the solution conductance (due to solution resistance).
3. A further requirement is to maximize the dynamic range of the output measurement (minimize the switching of ranges) to facilitate computer-controlled operation.
4. A further requirement is that the conductance output signal be essentially immune to the effects of ambient temperature changes to the circuit components over an operating range of 0 to 55 degrees Centigrade.
3. Prior Art
Background Information
Conventional conductance circuits utilize circuitry which applies alternating current to the electrodes to minimize the effects of electrode polarization and thus meet requirement No. 1 above. Also, conventional conductance measurement circuits ignore the effects of capacitive load components by exciting the load with square current signals and sampling the voltage after the capacitive components are charged and thus meet requirement #2 above.
The dynamic range of measurement of conventional circuits, however, is limited because of the use of linear gain circuitry which requires switching of several ranges. In addition, conventional circuitry will generally provide for operation at typically two different frequencies, but generally the frequency change must be effected in discrete steps along with switching of ranges.
A basic circuit which produces an output proportional to conductance is shown in FIG. 2, in which AR1 is an operational amplifier, R.sub.T is a resistance whose conductance is to be measured, R is a fixed resistance, e.sub.i is a fixed input DC voltage, and e.sub.o is the output voltage. The expression for the output voltage is ##EQU1## The conductance, G.sub.T, of the test resistance R.sub.T is ##EQU2## Combining eqs. (1) and (2) we obtain ##EQU3## If e.sub.i and R are constants, and k=e.sub.i R, equ. (3) becomes EQU e.sub.o =kG.sub.T ( 4)
which shows that the output voltage e.sub.o is proportional to the conductance G.sub.T of the resistance R.sub.T under test. Measurement of the conductance of R.sub.T in FIG. 2 is accomplished by applying a known, fixed value of DC current through R.sub.T and developing a voltage, e.sub.o, proportional to its conductance. If R.sub.T is the resistance of a solution in which electrodes are used to apply current as in FIG. 2, then, in order to minimize electrode polarization effects, the current through R.sub.T must be made to move in alternate directions at typical rates of a few hundred to a few thousand Hertz. When electrodes are used to measure the conductance of a solution, a capacitive component commonly appears along with the resistive component, the capacitive component being due to the interaction of the electrode with the ionized fluid in the solution under test.
One commonly used circuit configuration for applying alternating current to a solution under test and producing an output proportional to conductance which depends upon only the resistive component, R.sub.T, and not the capacitive component, C.sub.T, is shown in FIG. 3a. The input voltage, e.sub.i, is now a squarewave of typically a few hundred to a few thousand Hertz (FIG. 3b), which causes a corresponding squarewave of alternating current to flow through R.sub.T and C.sub.T. The output of AR1 (FIG. 3c), is a positive and negative exponential voltage due to the charging of C.sub.T alternately toward the potentials -Vo and +Vo. Without the capacitor C.sub.T, the output waveform would be as shown in dashed lines in FIG. 3c. If the amplitude of the output waveform is sampled after the capacitance is fully charged, only the effect of R.sub.T will be measured, and C.sub.T will be ignored. Switches S1 and S2 are activated at times t1 and t2, respectively, (FIGS. 3d and 3e), and AR2 causes inversion of the negative sampled voltage. A voltage, e.sub.o (FIG. 3f), proportional to only the resistive portion of the conductance of the solution, appears at hold capacitor C1. Buffer amplifier AR3 provides a low impedance output.
Limitation of Previous Method
The circuit of FIG. 3a produces an output proportional to conductance, but only over a limited range of conductances; a practical range of operation for this circuit might be over an output range of e.sub.o from +30 mV to +10 V, or a conductance measurement range of 333-to-1. To operate over a 5-decade range, or a 100,000-to-1 dynamic range, some means of range switching would be required, such as changing the value of R in FIG. 3 (Ra and Rb) so that e.sub.o would always stay within +30 mV to +10 V. For example, if e.sub.i were assumed to be +/-1 V, a required total measurement range of 0.01 uS to 1000 uS, and e.sub.o =30 mV to 10 V, two different values of R would be required to be switched into the circuit of FIG. 3, as shown in Table I:
TABLE I ______________________________________ Conductance Range Resistance Range Value of R ______________________________________ 3 uS to 1000 uS 1 k to 333 k 10 k .01 uS to 3 uS 333 k to 100M 3.3M ______________________________________
It is therefore an object of this invention to improve the prior art and provide the following:
1. Ability to measure conductance in one continuous five decade range which may be read out in two overlapping ranges to reduce the dynamic range requirements of a practical data acquisition system,
2. Automatic range indication to facilitate computer-controlled data acquisition,
3. Temperature compensation inherent in the design,
4. Optimum operating frequency automatically selected according to conductance value being measured.